The present invention relates to a perpendicular magnetic recording medium, a method of manufacturing the same, and a magnetic storage apparatus. More specifically, the invention relates to a magnetic recording medium having an areal recording density of 50 gigabits or more per square inch, the method of manufacturing the magnetic recording medium, and a magnetic storage apparatus that incorporates the magnetic recording medium thereinto.
Since 1998, the areal recording density of magnetic hard disk drives has been expanding at an annual increase rate of 100%. However, as the areal recording density is increased, a so-called thermal decay begins to be manifest. The thermal decay is a phenomenon where magnetically recorded data disappears due to the influence of ambient heat. Thus, the conventional longitudinal recording method is considered to be difficult to achieve the areal recording density exceeding 50 gigabits per square inch.
A perpendicular recording method, on the other hand, is different from the longitudinal recording method: this method possesses a feature where as a linear recording density is increased, a demagnetizing field between adjacent bits weakens, thereby maintaining recorded magnetization with stability. Further, by providing a soft magnetic underlayer with a high permeability under a perpendicular recording layer, a stronger head magnetic field is obtained. For this reason the coercivity of the perpendicular recording layer can be increased. For the above-mentioned reasons, the perpendicular recording method is considered to be one of promising means that can surmount the thermal fluctuation limit of the longitudinal recording method.
In the perpendicular recording method, a combination of a double-layer perpendicular recording medium constituted by the soft magnetic underlayer and the perpendicular recording layer with a single-pole-type head is effective in realizing high-density recording. However, since the double-layer perpendicular recording medium includes the soft magnetic underlayer with a high saturation magnetic flux density (Bs), a stray field generated from the domain wall of the soft magnetic underlayer is observed as spike noise. Further, a problem has been pointed out that recorded magnetization disappears due to movement of the domain wall of the soft magnetic underlayer. In order to solve the problems described above, there is proposed a method of providing a hard magnetic pinning layer between the soft magnetic layer and a substrate, thereby aligning magnetization of the soft magnetic underlayer in one direction, as disclosed in JP-A-7-129946 and JP-A-11-191217. There is also proposed a method of constituting the soft magnetic underlayer by a plurality of soft magnetic layers mutually separated by non-magnetic layers, thereby performing magnetization reversal between adjacent soft magnetic layers, as disclosed in JP-A-2001-155322.
In the method of providing the hard magnetic pinning layer, however, magnetic domains tend to be formed at the inner and outer edges of a disk, so that the spike noise might be observed from the magnetic domains. In addition, the process of magnetizing the hard magnetic pinning layer is required, which would raise manufacturing costs. In the method of performing magnetization reversal between adjacent soft magnetic layers, on the other hand, each layer tends to assume a multi-domain structure when the substrate is in the shape of a disk. Thus, it cannot be recognized to be effective in solving the problem of disappearance of recorded magnetization resulting from the domain wall motion.
JP-A-2002-074648 also discloses a method of employing ferromagnetic nanocrystals precipitated by annealing as a material for forming the soft magnetic underlayer. With this method, a clear domain wall is hard to be formed, so that this method is effective in reducing the spike noise and solving the problem of disappearance of recorded magnetization. However, depending on the process of forming a medium, weak spike noise or modulation of an output signal is sometimes observed. Such a reduction in the quality of the output signal becomes a problem when the magnetic hard disk drive is constructed.